The caffeine-carbon dioxide breath test may be a useful nonivasive measure of oxidative drug metabolism and liver function in alcoholic patients. 13-C-labeled methyl groups in caffeine are removed and further oxidized to 13-CO-2, which was measured in expired breath samples by isotope ratio mass spectrometry. Single 100mg doses of caffeine were administered to 10 alcoholic patients, at intervals from 2 to 73 days after the patients' last alcoholic drink. The breath test results were expressed as the cumulative amount of 13-CO-2 expired in 2 hrs. The results of tests in 6 smokers and 4 nonsmokers were compared and the changes in results in each patient were examined. The results of the first test (2 to 7 days after the last drink) were, on the average, 3.8 times higher in smokers than in nonsmokers (p<0.01),indicating more rapid metabolism in smokers. Over time, the breath test results increased more than 20% in 2 smokers and 3 nonsmokers, but decreased more the 20% in test result was in-creased to >200% of the first measurement and in 2 patients to 150% - 200%. The maximum decrease observed was to 57% of the first measurement. It appears that this test can be used to detect clinical alcoholics.